**Question:** Cafe-au-lait spots commonly occur when a person is

A. homozygous for the MC1R gene mutation
B. homozygous for the MC2R gene mutation
C. homozygous for the MC3R gene mutation
D. homozygous for the MC4R gene mutation

**Correct Answer:** A. homozygous for the MC1R gene mutation

**Core Concept:** Cafe-au-lait spots are benign, pigmented macules that appear due to increased melanin production in the skin. These spots are commonly associated with the presence of certain gene mutations and can be used as indicators of underlying genetic disorders.

**Why the Correct Answer is Right:** Cafe-au-lait spots are known to be associated with homozygous mutations in the melanocortin receptor 1 gene (MC1R). The MC1R gene is involved in melanin production regulation, and mutations in this gene lead to increased melanin production resulting in the formation of cafe-au-lait spots.

**Why Each Wrong Option is Incorrect:**

A. Homozygous mutations in the melanocortin receptor 2 gene (MC2R) and melanocortin receptor 3 gene (MC3R) do not lead to cafe-au-lait spots, as these genes are involved in other physiological processes and do not affect melanin production directly.

B. Homozygous mutations in the melanocortin receptor 4 gene (MC4R) are associated with obesity and short stature but do not cause cafe-au-lait spots.

C. Homozygous mutations in the tyrosinase gene (TYR) lead to oculocutaneous albinism, a condition characterized by complete absence of melanin pigmentation in the skin, hair, and eyes. This is different from cafe-au-lait spots, which are caused by increased melanin production.

**Clinical Pearl:**

Understanding the association between cafe-au-lait spots and MC1R gene mutations is crucial for clinical practice, as these spots can be an indicator of underlying genetic disorders, such as neurofibromatosis type 1 and Noonan syndrome. Early recognition of these spots can prompt further evaluation and prompt management of these conditions.